A number of polyester polymers are known to be useful as hot-melt adhesives for structurally bonding metals, woods, plastics, and other materials. When these polyester polymers have molecular weights high enough to provide satisfactory cohesive and adhesive bond strength, their melt viscosities are so high that they cannot be applied by conventional application equipment, such as gear pumps or piston pumps, used widely in the packaging industry with conventional, low-viscosity, polyolefin-based adhesives. Reducing the molecular weight of the polyesters to lower levels severely reduces bond strength of the adhesives.
In the manufacture of packaging adhesives based on ethylene-vinyl acetate copolymer, paraffin wax or other wax additives are used to reduce the melt viscosity of the adhesive blend. However, none of these usual viscosity-reducing waxes can be used with polyester polymers to provide the desired melt viscosity because they are highly incompatible with polyesters and separate out of the mixture as low-viscosity liquids in two-phase systems.
U.S. Pat. No. 4,172,824 discloses blends of certain poly(ethylene terephthalate) copolymers containing adipic acid and 1,4-butanediol with selected benzoate ester plasticizers.
These blends have melt viscosities low enough that they can be applied with conventional application equipment for hot-melt adhesives. However, the polyester portion of these blends has a tendency to decrease in inherent viscosity when the blends are heated. For example, these blends are typically found to decrease in inherent viscosity about 0.3-0.4 dl/g after being heated to typical application temperatures for eight hrs.
U.S. Pat. No. 4,094,721 relates to polyesters of terephthalic acid, 1,4-butanediol and 1,6-hexanediol useful as adhesives.
Benzoic acid esters are known for use in polymers. U.S. Pat. No. 3,186,961 discloses the use of various aryl carboxylic acid esters, for example, diethylene glycol dibenzoate, triethylene glycol dibenzoate, etc., in aromatic polyesters of carbonic acid. U.S. Pat. No. 2,044,612 discloses the use of certain benzoates as plasticizers for plastics, including condensation products of polyhydric alcohols and polybasic acids. Canadian Pat. No. 919,190 and British Pat. No. 815,991 also disclose the use of benzoic acid esters as plasticizers for vinyl resins.